What does the phrase new ‘Testament’ really mean?

Luther:

A testament is the promise of someone who is going to die, in which he describes who gets what. There is a testator, an inheritance and a inheritor. In the New Testament Christ is the testator, the inheritance is the forgiveness of sins and the inheritor is everyone who believes.

He has announced His Testament at the Lord’s Supper with the words: ‘This is My body, which is broken, this is my blood which is shed’ (the testator) ‘for the forgiveness of sins’ (the inheritance) ‘for you and for many’ (the inheritors).